Esters of polyhydric alcohols, also known as polyol esters, find a variety of uses on a large scale in industry, for example as plasticizers or lubricants. The selection of suitable starting materials allows the physical properties, for example boiling point or viscosity, to be controlled, and the chemical properties, such as hydrolysis resistance or stability to oxidative degradation, to be taken into account. Polyol esters can also be tailored to the solution of specific performance problems. Detailed overviews of the use of polyol esters can be found, for example, in Ullmann's Encyclopaedia of Industrial Chemistry, 5th edition, 1985, VCH Verlagsgesellschaft, Vol. A1, pages 305-319; 1990, Vol. A15, pages 438-440, or in Kirk Othmer, Encyclopaedia of Chemical Technology, 3rd edition, John Wiley & Sons, 1978, Vol. 1, pages 778-787; 1981, Vol. 14, pages 496-498.
The use of polyol esters as lubricants is of great industrial significance, and they are used particularly for those fields of use in which mineral oil-based lubricants meet the requirements set only incompletely. Polyol esters are used especially as turbine engine and instrument oils. Polyol esters for lubricant applications are based frequently on 1,3-propanediol, 1,3-butanediol, 1,4-butanediol, 1,2-hexanediol, 1,6-hexanediol, neopentyl glycol, trimethylolpropane, pentaerythritol, 2,2,4-trimethylpentane-1,3-diol, glycerol or 3(4),8(9)-dihydroxymethyltricyclo[5.2.1.02,6]decane, also known as TCD alcohol DM, as the alcohol component.
Polyol esters are also used to a considerable degree as plasticizers. Plasticizers find a variety of uses in plastics, coating materials, sealing materials and rubber articles. They interact physically with high molecular weight thermoplastic substances, without reacting chemically, preferably by virtue of their swelling and dissolution capacity. This forms a homogeneous system, the thermoplastic range of which is shifted to lower temperatures compared to the original polymers, one result being that the mechanical properties thereof are optimized, for example deformation capacity, elasticity and strength are increased, and hardness is reduced.
In order to open up the widest possible fields of use to plasticizers, they must fulfill a series of criteria. They should ideally be odorless, colorless, and light-, cold- and heat-resistant. Moreover, it is expected that they are insensitive to water, comparatively nonflammable and not very volatile, and are not harmful to health. Furthermore, the production of the plasticizers should be simple and, in order to meet ecological requirements, avoid waste substances, such as by-products which cannot be utilized further and wastewaters comprising pollutants. A specific class of polyol esters (they are referred to as G esters for short) contains diols or ether diols as the alcohol component, for example ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,2-propylene glycol or higher propylene glycols. They can be prepared in different ways. In addition to the reaction of alcohol and acid, optionally in the presence of acidic catalysts, further processes are employed in practice to obtain G esters, including the reaction of diol with acid halide, the trans-esterification of a carboxylic ester with a diol, and the addition of ethylene oxide onto carboxylic acids (ethoxylation). In industrial manufacture, only the direct reaction of diol and carboxylic acid and the ethoxylation of carboxylic acids have become established as production processes, preference usually being given to the esterification of diol and acid. This is because this process can be performed with no particular complexity in conventional chemical apparatus, and it affords chemically homogeneous products. Compared to this, ethoxylation requires extensive and costly technical equipment. Ethylene oxide is a very reactive chemical substance. It can polymerize explosively and forms explosive mixtures with air within very wide mixing ranges. Ethylene oxide irritates the eyes and respiratory pathways, leads to chemical burns and to liver and kidney damage, and is carcinogenic. The handling thereof therefore entails extensive safety measures. Moreover, scrupulous cleanliness of storage apparatus and reaction apparatus has to be ensured, in order to rule out the formation of undesired impurities as a result of side reactions of the ethylene oxide with extraneous substances. Finally, the reaction with ethylene oxide is not very selective, since it leads to mixtures of compounds of different chain length.
The direct esterification of alcohols with carboxylic acids is one of the basic operations in organic chemistry. In order to increase the reaction rate, the conversion is typically performed in the presence of catalysts. The use of one of the reactants in excess and/or the removal of the water formed in the course of the reaction ensures that the equilibrium is shifted in accordance with the law of mass action to the side of the reaction product, i.e. of the ester, which means that high yields are achieved.
Comprehensive information regarding the preparation of esters of polyhydric alcohols, also including esters of ethylene glycols and fatty acids, and regarding the properties of selected representatives of these compound classes can be found in Goldsmith, Polyhydric Alcohol Esters of Fatty Acids, Chem. Rev. 33, 257 ff. (1943). For example, esters of diethylene glycol, of triethylene glycol and of polyethylene glycols are prepared at temperatures of 130 to 230° C. over reaction times of 2.5 to 8 hours. To remove the water of reaction, carbon dioxide is used. Suitable catalysts mentioned for the esterification of polyhydric alcohols are inorganic acids, acidic salts, organic sulfonic acids, acetyl chloride, metals or amphoteric metal oxides. The water of reaction is removed with the aid of an entraining agent, for example toluene or xylene, or by introducing inert gases such as carbon dioxide or nitrogen.
The production and the properties of fatty acid esters of the polyethylene glycols are discussed by Johnson (edit.), Fatty Acids in Industry (1989) Chapter 9, Polyoxyethylene Esters of Fatty Acids, and a series of preparative hints are given. Higher diester concentrations are achieved by the increase in the molar ratio of carboxylic acid to glycol. Suitable measures for removing the water of reaction are azeotropic distillation in the presence of a water-immiscible solvent, heating while passing through an inert gas, or performing the reaction under reduced pressure in the presence of a desiccant. When the addition of catalysts is dispensed with, longer reaction times and higher reaction temperatures are required.
Both reaction conditions can be made milder by the use of catalysts. In addition to sulfuric acid, organic acids such as p-toluenesulfonic acid and cation exchangers of the polystyrene type are the preferred catalysts. The use of metal powders, such as tin or iron, is also described. According to the teaching from U.S. Pat. No. 2,628,249, color problems in the case of catalysis with sulfuric acid or sulfonic acids can be alleviated when working in the presence of activated carbon.
A procedure in which esters of diethylene glycol and triethylene glycol and of caprylic acid are prepared without addition of catalyst is known from U.S. Pat. No. 2,469,446. The esterification temperature is in the range from 270 to 275° C. and the water of reaction is driven out by means of a carbon dioxide stream.
The reaction regime in which the addition of a catalyst is dispensed with generally works with a molar excess of the particular carboxylic acid, which, owing to its acidity, also acts as a catalyst.
For the removal of the water of reaction formed in the ester formation from the polyol and the carboxylic acids, various processes are known. For example, the water of reaction formed is distilled out of the reaction vessel together with the excess carboxylic acid and passed into a downstream phase separator in which carboxylic acid and water separate according to the solubility properties thereof. In some cases, the carboxylic acid used also forms an azeotrope with water under the reaction conditions and is capable of removing the water of reaction as an entraining agent. Also employed are azeotropic distillation in the presence of an added water-immiscible solvent, heating of the reaction mixture while passing an inert gas through, and the reaction of the polyol and carboxylic acid starting materials under reduced pressure or in the presence of a desiccant. Especially the removal of water by azeotropic distillation has been found to be useful for the establishment of the equilibrium in the preparation of polyol esters. According to the procedure known from DE 199 40 991 A1, the water-immiscible solvent which acts as an entraining agent and which should have a boiling point of less than 112° C. is added to the reaction mixture only on attainment of a temperature of at least 140° C.
The crude ester obtained after removal of the water of reaction and of excess unconverted starting materials, appropriately the carboxylic acid added in excess, can first be treated with an alkaline reagent, for example with an aqueous sodium carbonate or sodium hydroxide solution, in order to remove last residues of acidic constituents. After washing with water and treatment with bleaching earth and activated carbon, last traces of coloring and odorous substances can be removed by applying reduced pressure at elevated temperature.
Processes for working up crude polyol esters are known, for example, from U.S. Pat. No. 2,469,446 A1. In some cases, the treatment with bleaching agents and activated carbon has to be repeated more than once in order to obtain end products with satisfactory color properties.
For the workup of the crude ester, U.S. Pat. No. 5,324,853 A1 proposes removing the excess acid by passing through nitrogen or steam, adding an adsorbent, neutralizing residual organic acid with a base and filtering off solids obtained. The residual amounts of acid present in the filtrate are removed with the passage of steam or nitrogen with simultaneous application of a reduced pressure and recycled back into the esterification reaction. Solids obtained in the vacuum treatment are removed in a final fine filtration. According to the procedure known from DE 199 40 991 A1, the crude ester is dried after alkali treatment, for example by passing an inert gas through the product or applying reduced pressure and optionally additionally distilling under reduced pressure. To improve the color of polyol esters, WO 94/18153 A1 proposes a subsequent treatment with an aqueous hydrogen peroxide solution.
Owing to the quality criteria described at the outset for polyol esters, the process steps in the esterification stage with removal of the water of reaction and in the workup of the crude ester are very important process features, since the adjustment of these process steps significantly influences the sensory and optical properties of the end products. More particularly, high demands are made on the color properties, such as low color number and high color stability, of the polyol esters. The structure of the starting materials, the polyhydric alcohols and the acids, is, in contrast, crucial for the mechanical and thermal properties of the polymer materials plasticized with the polyol esters and influences the hydrolysis and oxidation stability of lubricants.
In the course of preparation of polyol esters, the carboxylic acid used in excess, for example an aliphatic monocarboxylic acid, is removed in the course of the workup process and recycled back into the esterification process. In the continuous process regime, the recycling is effected during the running process, whereas, in the batchwise process, the excess aliphatic monocarboxylic acid removed is first collected and reused in the next batch. For the economic viability of the esterification process, high reuse rates of the aliphatic monocarboxylic acid are desirable. However, this is opposed by the fact that, with increasing reuse, the acid quality suffers as a result of the formation and concentration of by-products, such that the aliphatic monocarboxylic acid recovered has to be at least partly finally discharged and replaced by fresh acid in the course of continuous or batchwise operation. For an economically viable process, it is, however, desirable to use the aliphatic monocarboxylic acid recovered as frequently as possible in the continuous esterification process or, in the case of batchwise operation, in the subsequent production batches, without the quality of the desired polyol ester suffering.